1. Field of the Invention
The present invention relates to a method for measuring the waveform quality of a signal to be transmitted in a CDMA (Code Division Multiple Access) system.
2. Description of the Related Art
A conventional method of this type is described, for example, in a thesis entitled xe2x80x9cOverview of Code-Domain Power, Timing, and Phase Measurementsxe2x80x9d, Hewlett-Packard Journal, pp. 73-93, February 1996. In this method, as shown in FIG. 5, a received signal to be measured which has been converted to an intermediate frequency signal is inputted to a base band converting part 12 from an input terminal 11 and is converted to a base band signal. On the other hand, a diffusion code series from an input terminal 13 is inputted to a reference signal creating part 14 and a reference signal is created. Then a time base is extracted by a time base extracting part 15 from the reference signal and the base band signal from the base band converting part 12. The converted base band signal to be measured is corrected by a correcting part 16 based on the extracted time base. Then a frequency error and an initial phase of a carrier wave are estimated by a rough estimating part 17 from the corrected base band signal and the created reference signal. The corrected base band signal from the correcting part 16 is corrected by a correcting part 18 based on the estimated frequency error and the estimated initial phase. A frequency error and an initial phase of the carrier wave are further estimated by a close estimating part 19 based on the corrected base band signal and the diffusion code series from the terminal 13. The base band signal from the correcting part 16 is corrected by a correcting part 21 based on the estimated values of the frequency error and the initial phase of the carrier wave. From the corrected base band signal and the diffusion code series from the terminal 13, a waveform quality including a code domain power coefficient (a power ratio to all signals in each channel) is calculated by a measurement item calculating part 22.
In the conventional method described above, since a time base is directly extracted from a base band signal which is converted from a signal to be measured, there is a danger that the extracted time base is in error when a carrier frequency error is large to some extent. According to the above literature, when a signal length to be used for a time base extraction is 1.25 ms, the allowable frequency error is 400 Hz.
It is an object of the present invention to provide, even if a carrier frequency error of is equal to or more than 400 Hz, a method which can correctly extract a time base and hence can correctly measure a waveform quality.
In order to accomplish the above object, in an aspect of the present invention, there is provided a method for measuring a CDMA signal comprising: an orthogonal converting step for converting a digital signal to be measured into a first complex base band signal; a time base/frequency error estimating step for estimating a first signal delay value from the first complex base band signal and a diffusion code series and for estimating a first frequency error utilizing the estimated first signal delay value; a frequency error correcting step for correcting the first complex base band signal by the first frequency error and for shifting the first complex base band signal by the first signal delay value to obtain a second complex base band signal; a reference signal creating step for creating a reference signal from the second complex base band signal and a diffusion code series; a parameter estimating/correcting step for estimating and correcting a parameter from the second complex base band signal and the reference signal to obtain a third complex base band signal; and a measurement item calculating step for calculating a measurement item from the third complex base band signal and the reference signal.
In a preferred embodiment, the orthogonal converting step includes a sampling rate converting step for converting a complex base band signal into a sampling frequency having n time frequency (n is an integer number) of its chip rate to obtain the first complex base band signal.
The time base/frequency error estimating step comprises: a symbol point extracting step for detecting a sampling point closest to a symbol point from the first complex base band signal to estimate the first signal delay value; and a synchronizing/frequency error estimating step for detecting, for each of the extracted symbol points, a phase of a diffusion code series from the first complex base band signal and a diffusion code series to estimate the first frequency error from the phase difference.
The synchronizing/frequency error estimating step comprises: a long code mask detecting step for detecting a long code mask portion from the first complex base band signal and a diffusion code series; and an estimating step for detecting a phase of a long code following the long code mask portion from the first complex base band signal and a diffusion code series to estimate the first frequency error from the phase difference.
The reference signal creating step comprises: a demodulating/power estimating step for demodulating each channel from the second complex base band signal and a diffusion code series of each channel and for estimating a first power value of each channel; and a reference signal generating step for creating a reference signal of each channel from the diffusion code series of each channel and the first power value of each channel.
The parameter estimating/correcting step comprises: a second symbol point measuring step for estimating a second signal delay value from the second complex base band signal and the reference signal; a signal delay correcting step for shifting the second complex base band signal by the second signal delay value to obtain the third complex base band signal; a frequency error/initial phase estimating step for estimating a second frequency error and an initial phase of a carrier wave; a frequency error/initial phase correcting step for correcting the third complex base band signal by the second frequency error and the initial phase value to obtain a fourth complex base band signal; and an amplitude estimating step for estimating an amplitude of each channel from the fourth complex base band signal and the reference signal.
The second symbol point measuring step and the signal delay correcting step are used a plurality of times in the parameter estimating/correcting step.
According to the present method, even if an input signal (a digital signal to be measured) should include a somewhat large frequency error, a time base can be extracted and hence a code domain power coefficient and a waveform quality of a signal to be measured having a relatively large frequency error can correctly be measured.